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On the surface it's a minor change, and yet it's significant in reflecting the shifting focus and user behavior of the service over time. As most users know, the official question is largely ignored by those who have found myriad ways to share pretty much anything they wanted, be it information, relationships, entertainment, citizen journalism, and beyond.
The change acknowledges that Twitter has grown far beyond the more personal status updates it was originally envisioned to convey, and has morphed into a sort of always-on, source-agnostic information network that is wholly unique. Twitter says they don't expect the change to at all influence how people actually use Twitter, but might "make it easier to explain to your dad."